A salamander that never grows up can regrow its legs, heart, and parts of its brain

Most salamanders begin life in the water as gilled larvae, then transform into air-breathing adults that crawl onto land. The axolotl never makes that change. It stays in the water its whole life, keeping the feathery external gills of a juvenile even as it matures and breeds, a permanent adolescent that looks perpetually young. That refusal to grow up is only the beginning of what makes it extraordinary.

The axolotl can regrow entire limbs, sections of its heart, portions of its eyes and even parts of its brain, rebuilding complex tissue that most animals, including humans, can only scar over. This regenerative power has made the small Mexican amphibian one of the most intensively studied creatures in biology, even as its wild population teeters on the edge of extinction in the shrinking waterways it calls home.

A salamander stuck in childhood

The axolotl, scientifically Ambystoma mexicanum, is a species of mole salamander that exhibits neoteny, meaning it reaches sexual maturity without undergoing metamorphosis. Where relatives such as the tiger salamander lose their gills and leave the water as adults, the axolotl remains fully aquatic, retaining three pairs of external gill stalks that fan out behind its wide, lidless head. According to the biology of the axolotl, a mature adult typically ranges from about 15 to 45 centimeters long, with sizes near 23 centimeters most common. Its permanently larval appearance, wide-mouthed and seemingly smiling, has helped make it a fixture of aquariums and popular culture.

The regeneration that draws scientists

The trait that most captivates researchers is the axolotl’s healing ability. It can regenerate lost limbs completely, along with gills and portions of the eyes and brain, regrowing not just skin but bone, muscle, blood vessels and nerves in the correct arrangement. The regenerated structures are functional, and the animal can repeat the feat many times over the course of its life. That capacity has turned the axolotl into a model organism for studying tissue regeneration, as scientists work to understand how it can rebuild body parts that mammals cannot, and whether any of those mechanisms might one day inform human medicine.

Why it heals without scars

A key to the axolotl’s regeneration is that it does not respond to injury the way most animals do. Instead of sealing a wound with scar tissue, it forms a specialized mass of cells at the injury site that can rebuild the missing structure from scratch. This scar-free healing lets the regrowth proceed without the fibrous barrier that, in mammals, blocks the reconstruction of complex tissue. Understanding how the axolotl reactivates the developmental programs needed to grow a limb, and does so in an adult body, is a central question for the field of biological regeneration, and it is the reason large captive colonies are maintained in laboratories worldwide.

An enormous genome

The axolotl’s biology extends to its DNA. Its genome, sequenced and published in 2018, runs to some 32 billion base pairs, roughly ten times the size of the human genome and, at the time, the largest animal genome fully assembled. Yet it encodes a similar number of proteins to humans, around 23,000; the difference in size comes largely from vast stretches of repetitive sequence and unusually long stretches within genes. Analyzing that genome has helped researchers begin to identify species-specific genetic pathways that may underlie the animal’s regenerative talents, providing a molecular map to accompany decades of laboratory observation.

The vanishing lakes of Xochimilco

For all its scientific prominence, the wild axolotl is in grave danger. The species originally lived in a network of interconnected lakes and wetlands in the highlands of central Mexico, chiefly the waters of Xochimilco and Chalco. The draining of those lakes, begun in the era of the Aztecs and vastly accelerated during the 20th century, has destroyed most of that habitat, much of it now buried beneath Mexico City. Invasive fish such as tilapia and carp, which prey on axolotls and their eggs, have driven the wild population down to an estimated 50 to 1,000 adults, and the species is now assessed as critically endangered.

A survivor between two fates

The axolotl thus occupies a paradoxical position. In laboratories and home aquariums it flourishes in the tens of thousands, valued for its resilience, its regenerative powers and its striking color variants, from the pale, pink-bodied leucistic form to albino and near-black melanistic types. In the wild, the animal that so readily regrows its own body cannot outpace the loss of its habitat. Its name traces to Xolotl, an Aztec deity associated with fire, lightning and the dead, an apt namesake for a creature that seems to defy the ordinary rules of growth and repair, even as its native waters disappear.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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